Vesicle transport is the general process in eukaryotic cells by which proteins synthesized in the endoplasmic reticulum (ER) are transported via the Golgi network to the various compartments in the cell where they will function. Other proteins are transported to the cell surface by this process where they may be secreted (exocytosis). Such proteins include membrane bound receptors or other membrane proteins, neurotransmitters, hormones, and digestive enzymes. The transport process uses a series of transport vesicles that shuttle a protein from one membrane-bound compartment (donor compartment) to another (acceptor compartment) until the protein reaches its proper destination (Rothman, J. E and Wieland, F. T. (1996) Science 727:227-34).
The process begins with the budding of a vesicle out of the donor membrane. The vesicle contains the protein to be transported and is surrounded by a protective coat made up of protein subunits recruited from the cytosol. The initial budding process and coating processes are controlled by a cytosolic GTP-binding protein, either SAR or ARF. When GTP binds and activates SAR, it binds to the donor membrane and initiates the vesicle assembly process. The coated vesicle containing the GTP-SAR complex detaches from the donor compartment and is transported through the cytosol. During the transport process, the SAR-bound GTP is hydrolyzed to GDP, and the inactivated SAR dissociates from the transport vesicle. At this point, the protective coat becomes unstable and dissociates from the enclosed vesicle. The uncoated vesicle is recognized by its acceptor compartment through exposed surface identifiers (v-SNAREs) which bind with corresponding molecules on the acceptor compartment membrane (t-SNAREs). The transport process ends when the vesicle fuses with the target membrane.
The fusion of the transport vesicle with the acceptor compartment membrane, that follows the initial binding of the two compartments, involves the formation of a complex between the v-SNARE, t-SNARE, and certain other proteins recruited from the cytosol. Many of these other proteins have been identified although their exact functions in the fusion complex remain uncertain (Tellam, J. T. et al. (1995) J. Biol. Chem. 270:5857-63; Hata, Y. and Sudhof, T. C. (1995) J. Biol. Chem. 270:13022-28). N-ethylmaleimide sensitive factor (NSF) and soluble NSF-attachment protein (SNAP) are two such proteins that are conserved from yeast to human and function in most intracellular membrane fusion reactions.
Neurotransmission in mammals involves a specialized form of vesicle transport which uses a signaling molecule (neurotransmitter) stored in a membrane-bound vesicle (synaptic vesicle) at the terminus of a nerve cell. A change in electrical potential at the nerve terminal results from excitation of the nerve and triggers the release of the neurotransmitter from the synaptic vesicles by exocytosis. The neurotransmitter rapidly diffuses across the junction (synaptic cleft) separating the first (presynaptic) nerve cell from the second (postsynaptic) and provokes a change in electrical potential in the latter by binding to and opening transmitter-gated ion channels located in the plasma membrane of the postsynaptic cell. In this manner, the neural signal is transmitted from one nerve cell to the other.
Many of the proteins involved in synaptic vesicle transport have been identified and the biochemical interactions between them have been characterized. Interestingly, many of these proteins are homologous to yeast proteins involved in yeast secretory pathways (sec). For example, yeast Sec4 is a GTPase essential for late stages of vesicle secretion and is homologous to mammalian Rab3a GTPase. Yeast Sec6p and Sec8p are components of a 19.5S particle that interact with Sec4. Mammalian counterparts to these two proteins, rSec6 and rSec8, have been identified from rat brain cDNA libraries (Ting, A. E. et al. (1995) Proc. Natl. Acad. Sci. 92:9613-17). rSec6 is an 87 kDa protein with 22% amino acid identity to Sec6, and rSec8 is a 110 kDa protein with 20% identity to Sec8. Further studies revealed that rSec6 and rSec8 are components of a 17S complex in rat brain that is composed of 8 proteins with a combined molecular weight of 743 kDa (Hsu, S-C et al. (1996) Neuron 17:1209-19). The rSec6/rSec8 complex coimmunoprecipitates with syntaxin, a plasma membrane t-SNARE protein involved in neurotransmission. This indicates a role for the rSec6/rSec8 complex in the neurotransmission process. rSec6 and rSec8 are similarly expressed in all regions of rat brain, suggesting that both proteins are required by all brain cells (Hsu et al. supra). In addition, both proteins are found in a variety of non-neuronal tissues including lung, muscle, ovary, and kidney, indicating that they may function in a variety of secretory pathways in addition to neurotransmission (Ting et al. supra; Hsu et al. supra).
Few structural motifs have been identified that characterize these vesicle transport proteins. However, both rSec6 and rSec8 contain a region near their N-terminus capable of forming coiled-coil domains, a motif frequently observed in vesicle trafficking proteins and believed to be important for interactions with other proteins (Ting et al. supra). rSec8 also contains a leucine-rich domain near its N-terminus that is postulated to interact with GTP-binding proteins such as Rab3a.
The control of vesicle transport processes, particularly the process of neurotransmission, has important implications for the control of various diseases and disorders. Current drug treatments for the brain disorder schizophrenia target receptors for various neurotransmitters such as dopamine and serotonin (Bunk, S. (1997) The Scientist 11:1-5). However these treatments have numerous unwanted side effects.
The discovery of a new human rSec6 related protein and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of cancer, immune disorders, and neurodegenerative disorders.